1. Field of the Invention
The present invention relates to an operational amplifier circuit having fast-response characteristics and a high-frequency operational range. The invention also relates to a constant voltage circuit that employs such an operational amplifier circuit, and an apparatus that employs such a constant voltage circuit.
2. Description of the Related Art
Conventionally, in operational amplifier circuits and constant voltage circuits employing an operational amplifier circuit, metal-oxide semiconductor (MOS) transistors are used that are advantageous in reducing current consumption. In order to increase the withstand voltage of a MOS transistor, generally the gate oxide film is made thicker to prevent gate oxide film breakdown, or the channel length L is increased to prevent punch-through. Such MOS transistors, however, have limited current drive capacity and are not suitable for handling large currents. On the other hand, bipolar transistors are capable of handling large currents by controlling their base current even when their withstand voltages are increased. Generally, bipolar transistors are mainly used in constant voltage circuits partly because they are capable of driving large currents at a high withstand-voltage and ensuring a high ripple removal ratio in a high-frequency band. However, MOS transistors need to be used in order to reduce current consumption.
FIG. 1 shows an example of a conventional constant voltage circuit composed of MOS transistors.
In this circuit, in order to enable a high speed operation, an operational amplifier circuit 101 that controls the operation of a driver transistor M101 has a multi-stage amplifier configuration consisting of a differential amplifier circuit AMPa, which is a differential amplifying stage, and an amplifier circuit AMPb, which is an output stage. In this configuration, the gate voltage of the driver transistor M101 is controlled by the amplifier circuit AMPb in response to a shift in an output voltage Vout. Thus, to the gate of the driver transistor M101 is applied a voltage ranging from a ground voltage GND to an input voltage Vdd. Similarly, the output voltage of the differential amplifier circuit AMPa ranges from the ground voltage GND to the input voltage Vdd. As a result, PMOS transistors M112 and M115 need to have high withstand voltages.
Thus, constant voltage circuits that handle high voltages, such as 36 V, generally employ high withstand-voltage MOS transistors. Because high withstand-voltage MOS transistors have a thick gate oxide film and a large channel length L, these transistors have low current drive capacity, making them unsuitable for use in a constant voltage circuit that needs to output large currents and have a fast response.
The aforementioned problem has been dealt with by using laterally diffused MOS (LDMOS) transistors as a driver transistor in a constant voltage circuit (see Japanese Patent No. 3683185, for example). However, in order to realize a fast response, a high withstand-voltage transistor needs to be used to drive the gate of the LDMOS transistor.
If a high withstand-voltage transistor is used as the transistor M115 for driving the gate of the driver transistor M101 in the circuit of FIG. 1, increasing the constant current i102 in order to realize a fast response naturally makes it necessary to increase the size of the PMOS transistor M115. Furthermore, in order to stabilize circuit operation during a steady-state operation, the differential amplifier circuit AMPa and the amplifier circuit AMPb need to have the same operating point in order to align their outputs. This means that the PMOS transistors M111 and M112 need to have the same size as the PMOS transistor M115. As a result, the channel widths W of the PMOS transistors M111, M112, and M115 have to be increased, which results in an increase in drain junction capacity. This is disadvantageous in terms of fast response characteristics.
Because the PMOS transistors M111, M112, and M115 have relatively large channel length L, their output resistance is large. As a result, the pole of the operational amplifier circuit 101 tends to be positioned at the lower frequency side, thus making operation in a high-frequency range disadvantageous. In a steady-state operation, the driver transistor M101 has a sufficient current drive capacity for an output current iout outputted at an output terminal OUT, so that the gate-source voltage of the driver transistor M101 does not become too high.
Thus, in a constant voltage circuit in which an operational amplifier circuit of a multi-stage amplifier configuration is employed in order to achieve a fast response, if the circuit is composed of high withstand-voltage MOS transistors alone, the pole of the output of each amplifying stage of the operational amplifier circuit is positioned at the low-frequency side, making phase compensation difficult at small load. If the constant current supplied to each amplifying stage of the operational amplifier circuit is to be increased to achieve a fast response, the MOS transistors must have a large channel width W, whereby the overlap capacity between gate and drain and the drain junction capacity increase, which is disadvantageous for the realization of high-speed operation.